Characterization and expression of Arabidopsis UDP-sugar pyrophosphorylase.

At5g52560, a homolog of pea (Pisum sativum) UDP-sugar pyrophosphorylase (PsUSP) was functionally annotated by expression in Escherichia coli and subsequent characterization of substrate specificity and kinetic properties. Arabidopsis contains a single USP gene (AtUSP) and evaluation of gene databases suggests that USP is unique to plants. The 69 kDa AtUSP gene product exhibited high activity with Glc-1-P, GlcA-1-P and Gal-1-P, but low activity with GlcNAc-1-P, Fuc-1-P, Man-1-P, inositol-1-P or Glc-6-P. AtUSP was activated by magnesium and preferred UTP as co-substrate. Apparent K(m) values for GlcA-1-P, Glc-1-P and UTP were 0.13 mM, 0.42 mM and 0.14 mM, respectively. In the reverse direction (pyrophosphorolysis), the apparent K(m) values for UDP-GlcA, UDP-Glc and pyrophosphate were 0.56 mM, 0.72 mM and 0.15 mM, respectively. USP enzyme activity (UDP-GlcA --> GlcA-1-P) was detected in Arabidopsis tissues with highest activity found in the inflorescence. As determined by semi-quantitative RT-PCR, AtUSP transcript is widely expressed with high levels detected in the inflorescence. To evaluate tissue-specific expression of AtUSP, histochemical GUS staining of plants transformed with AtUSPprom:GUS constructs was performed. In 7-day-old seedlings, GUS staining was detected in cotyledons, trichomes and vascular tissues of the primary root. In the inflorescence of older plants, high levels of GUS staining were detected in cauline leaves, the epidermis of the stem and in pollen. In silico analysis of AtUSP expression in developing pollen indicates that transcript levels increase as development proceeds from the uninucleate to the tricellular stage. The results suggest that AtUSP plays an important role in pollen development in Arabidopsis.

Structural and functional correlates of a mutation in the malignant hyperthermia-susceptible pig ryanodine receptor.

The skeletal muscle ryanodine receptor of malignant hyperthermia-susceptible (MHS) pigs contains a mutation at residue 615 that is highly correlated with various abnormalities in the regulation of sarcoplasmic reticulum (SR) Ca2+ channel activity. In isolated SR membranes the Arg615 to Cys615 ryanodine receptor mutation is now shown to be directly responsible for an altered tryptic peptide map, due to the elimination of the Arg615 cleavage site. Furthermore, trypsin treatment released 86-99 kDa ryanodine receptor fragments encompassing residue 615 from the SR membranes. We conclude that the 86-99 kDa domain containing residue 615 is near the cytoplasmic surface of the ryanodine receptor and likely near important Ca2+ channel regulatory sites.

Reconstitution of abnormalities in the malignant hyperthermia-susceptible pig ryanodine receptor.

Malignant hyperthermia-susceptible (MHS) pigs homozygous for the Cys615 ryanodine receptor allele demonstrate altered sarcoplasmic reticulum (SR) ryanodine binding and Ca2+ release channel regulatory properties when compared with normal pigs homozygous for the Arg615 allele. While solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, the purified MHS and normal ryanodine receptors had a similar dissociation constant (Kd) for ryanodine, maximum binding, and Ca2+ concentration for half-maximal stimulation and inhibition of ryanodine binding (Ca2+(0.5)); however, after reconstitution into proteoliposomes, the purified MHS and normal receptors had Kd values for ryanodine of 75 and 150 nM, respectively, which were significantly different. The purified MHS and normal porcine ryanodine receptors also had similar single-channel Cs+ conductance, optimal cis-Ca2+ for channel opening, and cis-Ca2+(0.5) for channel activation. Significantly, at inactivating levels of cis-Ca2+ (> 0.1 mM), MHS channels had a greater open probability, a higher cis-Ca2+(0.5) for inhibition of channel opening (250 vs. 75 microM for MHS and normal, respectively), longer mean open times, and shorter mean closed times than did normal channels. We conclude that the mutation at residue 615 causes a detectable alteration in ryanodine receptor/Ca2+ channel activity and thus may represent the primary defect responsible for the altered SR Ca2+ regulation characteristic of MHS porcine muscle.

Stimulation and inhibition of [3H]ryanodine binding to sarcoplasmic reticulum from malignant hyperthermia susceptible pigs.

When compared to normal pig sarcoplasmic reticulum (SR), SR from malignant hyperthermia susceptible (MHS) porcine skeletal muscle has been shown to exhibit an increased rate of calcium release, as well as alterations in [3H]ryanodine-binding activity in the presence of microM Ca2+ (Mickelson et al., 1988, J. Biol. Chem. 263, 9310). In the present study, various stimulators (adenine nucleotides and caffeine) and inhibitors (ruthenium red and Mg2+) of the SR calcium release channel were examined for effects on MHS and normal SR [3H]ryanodine binding. The apparent affinity of the MHS SR receptor for ryanodine in the presence of 10 mM ATP (Kd = 6.0 nM) or 10 mM caffeine (Kd = 28 nM) was significantly greater than that of the normal SR (Kd = 8.5 and 65 nM in 10 mM ATP or caffeine, respectively), the Bmax (12-16 pmol/mg) was similar in all cases. The Ca2+(0.5) for inhibition of [3H]ryanodine binding in the presence of 5 mM AMPPNP (238 vs 74 microM for MHS and normal SR, respectively) and the Ca2+(0.5) for stimulation of [3H]ryanodine binding in the presence of 5 mM caffeine (0.049 vs 0.070 microM for MHS and normal SR, respectively) were also significantly different. Furthermore, in the presence of optimal Ca2+, MHS SR [3H]ryanodine binding was more sensitive to caffeine stimulation (C0.5 of 1.7 vs 3.4 mM) and was less sensitive to ruthenium red (C0.5 of 1.9 vs 1.2 microM) or Mg2+ inhibition (C0.5 of 0.34 vs 0.21 mM) than was normal SR. These results further support the hypothesis that differences in the ryanodine/receptor calcium release channel regulatory properties are responsible for the abnormal calcium releasing activity of MHS SR.

Abnormal sarcoplasmic reticulum ryanodine receptor in malignant hyperthermia.

Previous studies have demonstrated that skeletal muscle from individuals susceptible to malignant hyperthermia (MH) has a defect associated with the mechanism of calcium release from its intracellular storage sites in the sarcoplasmic reticulum (SR). In this report we demonstrate that the [3H]ryanodine receptor of isolated MH-susceptible (MHS) porcine heavy SR exhibits an altered Ca2+ dependence of [3H]ryanodine binding at the low affinity Ca2+ site as well as a lower Kd for ryanodine (92 versus 265 nM) when compared to normal porcine SR. The Bmax of the normal and MHS [3H] ryanodine receptor (9.3-12.6 pmol/mg) was not significantly different, and analysis of MHS and normal SR proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis did not reveal a significant difference in the intensity of Coomassie Blue staining of the spanning protein/ryanodine receptor region of the gels (Mr greater than 300,000). We also find that MHS porcine muscle intact fiber bundles exhibit a 5-10-fold lower ryanodine threshold for twitch and tetanus inhibition, and contracture onset when compared to normal muscle. Since the SR ryanodine receptor is a calcium release channel as well as a component intimately involved in transverse tubule-SR communication, abnormalities in the skeletal muscle ryanodine receptor may be responsible for the abnormal SR calcium release and contractile properties demonstrated by MHS muscle.

Skeletal muscle junctional membrane protein content in pigs with different ryanodine receptor genotypes.

The content of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, transverse tubule dihydropyridine receptor (DHPR), and SR ryanodine receptor (RyR) was determined in muscle of pigs homozygous for the normal RyR allele and homozygous or heterozygous for the malignant hyperthermia-susceptible (MHS) RyR allele. Total muscle membranes isolated from 1-day-old pigs of the three different genotypes did not differ in the content of any of these proteins. However, at 28 days of age, crude membranes and total muscle homogenates from homozygous MHS pigs exhibited only 61-81% of the [3H]PN 200-110 or [3H]ryanodine binding of identical preparations isolated from normal pigs; these MHS membranes also contained only 50% of the normal content of each of the DHPR subunits. The crude membranes and muscle homogenates from heterozygous pigs were intermediate to both types of homozygotes in terms of [3H]PN 200-110 binding, [3H]ryanodine binding, and the content of the DHPR subunits. However, membrane preparations enriched in triadic junctional proteins isolated from 3- to 4-mo-old pigs of the three different genotypes did not differ in their [3H]PN 200-110 binding, [3H]ryanodine binding, or Ca(2+)-ATPase activities. We conclude that, although the stoichiometry of the RyR to DHPR is not altered, the presence of the MHS RyR allele during muscle development results in a decreased relative content of these two proteins. This is probably due to a lower junctional membrane content and may be an important ultrastructural consequence of the altered sarcoplasmic Ca2+ regulation in MHS muscle.

Effects of propofol on Ca2+ regulation by malignant hyperthermia-susceptible muscle membranes.

BACKGROUND: The effects of inhalation anesthetics on Ca2+ regulation in malignant hyperthermia-susceptible skeletal muscle are considered to be responsible for triggering malignant hyperthermia. The intravenous anesthetic propofol does not trigger malignant hyperthermia in susceptible patients or experimental animals, suggesting that there are important differences between the effects of propofol and the effects of inhalation anesthetics on Ca2+ regulation in malignant hyperthermia-susceptible muscle. Understanding these differences may help to clarify the mechanisms responsible for triggering malignant hyperthermia. METHODS: To investigate the effects of propofol on Ca2+ regulation by malignant hyperthermia-susceptible skeletal muscle, we determined its effects on the membrane channels and pumps that control myoplasmic Ca2+ concentrations: the sarcoplasmic reticulum ryanodine receptor, the transverse tubule dihydropyridine receptor, and the sarcoplasmic reticulum Ca(2+)-adenosine triphosphatase (Ca(2+)-ATPase). Terminal cisternae-derived sarcoplasmic reticulum vesicles enriched in the junctional proteins of the sarcoplasmic reticulum and the transverse tubule membranes were isolated from the muscle of malignant hyperthermia-susceptible and normal pigs. Ca2+ flux, Ca(2+)-ATPase, and ligand binding measurements on these isolated vesicle preparations were performed in the presence of varying propofol concentrations. RESULTS: Propofol (10-500 microM) had no effect on ryanodine receptor-mediated Ca2+ efflux from muscle membrane vesicles. Propofol (1-100 microM) also had no effect on sarcoplasmic reticulum vesicle [3H]ryanodine binding, whereas higher concentrations (200-300 microM) slightly inhibited [3H]ryanodine binding. Binding of the dihydropyridine receptor Ca2+ channel blocker [3H]PN200-110 to these preparations was inhibited by propofol (10-300 microM). Ca(2+)-ATPase activity was stimulated by 10-100 microM propofol but was inhibited by higher concentrations. In all cases, the effects of propofol on malignant hyperthermia-susceptible and normal membrane preparations were similar. CONCLUSIONS: In contrast to malignant hyperthermia-triggering inhalation anesthetics, propofol does not stimulate malignant hyperthermia-susceptible or normal ryanodine receptor channel activity, even at > 100 times clinical concentrations. Effects on dihydropyridine receptor and Ca(2+)-ATPase function, however, are similar to the effects of inhalation anesthestics and require much lower concentrations of propofol. These findings, demonstrating that propofol does not activate ryanodine receptor Ca2+ channels, suggest a plausible explanation for why propofol does not trigger malignant hyperthermia in susceptible persons.

Effects of the halothane-sensitivity gene on sarcoplasmic reticulum function.

Pigs heterozygous for the halothane-sensitivity gene exhibit a distinct phenotype with regard to both in vivo and in vitro muscle responses to halothane (E. M. Gallant, J. R. Mickelson, B. D. Roggow, S. K. Donaldson, C. F. Louis, and W. E. Rempel. Am. J. Physiol. 257 (Cell Physiol. 26): C781-C786, 1989). In this paper heavy sarcoplasmic reticulum (SR) preparations were isolated from the muscles of pigs of all three genotypes. The rate of calcium release from SR of pigs homozygous for the halothane-sensitivity gene was approximately twice that of SR from pigs homozygous for the normal allele. Furthermore, in the presence of 6 microM Ca2+, the binding of [3H]ryanodine to SR isolated from the homozygous halothane-sensitive pigs was of a higher affinity than was the binding to SR isolated from the homozygous normal pigs (Kd = 70-90 vs. 265 nM, respectively). The SR from pigs heterozygous for the halothane-sensitivity gene, however, demonstrated intermediate values for the rate of calcium release and the affinity for [3H]ryanodine (Kd = 192 nM). Thus the alterations in heavy SR calcium release and [3H]ryanodine binding in the pigs containing one copy of the halothane-sensitivity gene demonstrate a distinct heterozygote phenotype. These data also suggest that the protein product of this gene is closely associated with, and perhaps identical to, the SR calcium release channel-ryanodine receptor protein.